The present invention relates to a range finder for use in a camera or the like for automatically measuring a distance to a subject in photographing.
As an active range finder, there has been proposed a system such as that shown in FIG. 1.
Light 10a emitted from a light source 10, for example, an infrared-ray source or the like, is projected onto a subject 14 through a lens system 12, and reflected light 10b from the subject 14 is projected onto a light detecting element 18 through a lens system 16 so as to form a light spot.
The light detecting element 18 functions as a position-sensitive detector, that is, a position detector, in which the values of currents I.sub.1 and I.sub.2 obtained from electrodes 18a and 18b, respectively, vary depending on the position of the light spot. On the basis of this fact, an angle .theta. between the respective optical axes of the projection light 10a and the reflected light 10b is obtained from the ratio of the current I.sub.1 to the current I.sub.2, and the distance to the subject 14 is determined by the angle .theta..
However, if the light spot formed on the light detecting element 18 is weak, the processing operation of a distance operation circuit cannot be performed because the noise level will be too high, and therefore it is necessary to make the respective values of the currents I.sub.1 and I.sub.2 large enough to ensure that calculating processing can always be carried out. A large current is therefore always supplied to the light source 10 in ranging so as to cause the light source 10 to emit intensive light, the light intensity being determined taking into consideration the worst-case ranging conditions, namely, where the subject 14 is far and the light reflection factor is low.
In order to perform ranging, however, it is a sufficient condition to obtain the ratio of the current I.sub.1 to the current I.sub.2. Hence, the absolute values of the currents I.sub.1 and I.sub.2 are not required. It is therefore wasteful to cause the light source 10 to emit intense light under good ranging conditions where the subject 14 is near and the reflection factor of the subject is high because of the useless consumption of electric power. Particularly in the case where a battery is used as the power supply, there has been a problem in that the rate of consumption of the battery is so large that the number of times or cycles a ranging operation can be carried out without changing the battery is small.
There is a further problem in that if the light source 10 is always caused to emit intense light, the light source 10 is apt to deteriorate, resulting in the generation of electrical noise due to the presence of a pulse-like large current. Such noise can adversely affect the operation of peripheral circuits. Noise often affects the distance calculation circuit to make its processing operation inaccurate.
In order to reduce the influence of electrical noise, multiple ranging operations are typically continuously and successively performed several or ten-odd times and the average of the determined distances is used as the final distance value. Ranging operations carried out successively through plural times or cycles are herein termed "one ranging". The time required for one ranging is generally about 100 msec.
The number of times or cycles in "one ranging" is fixedly set to a considerably large value taking into consideration the worst-case conditions where the subject 14 is far away and the reflection factor of the subject is low.
Under good ranging condition where the subject 14 is near and the reflection factor of the subject is high, however, the light spot on the light detecting element 18 is so intense as to increase the values of the currents I.sub.1 and I.sub.2 obtained from the light spot to such a level that the distance calculation circuit is hardly affected by noise. That is, one ranging operation would suffice to provide accurate ranging calculations. Therefore, to perform such a large number of times or cycles in "one ranging" under good ranging conditions is not only meaningless but wasteful in terms of electric power consumption. In cameras employing a high degree of automation, for example, automatic film winding, autofocusing, and the like, the amount of wasted power can be quite significant.
Further, the time taken for "one ranging" is unnecessarily long, so that in a camera in which autofocusing is performed using the average of plural ranging operations, there has been a problem that the focusing operation is delayed correspondingly.